Images such as photographs, x-rays, and maps contain an abundance of information. Often such images are recorded as electronic images to allow digital storage and indexing, rapid access, and post-processing including constructing a composite image from multiple existing images, filtering existing images, and removing noise from existing or constructed images.
Magnetic resonance imaging (MRI) is a widely-used diagnostic technique in which radio frequency (RF) signals are analyzed to produce diagnostic information and stored as electronic images. Echo-planar imaging (EPI) is a subset of MRI that provides high temporal resolution achieved with faster imaging techniques, and typically results in large image data sets. One application of EPI is functional magnetic resonance imaging (fMRI), where a time series of images are acquired for a selected plane (or planes) of a subject. In challenge-based fMRI of the human brain, a time series of images of one or more planes within a subject's brain are collected while the subject is exposed to a sequence of stimulus conditions, to identify functional changes in brain characteristics.
The high spatial resolution of EPI makes challenge-based experiments sensitive to subject movements on the scale of millimeters or less. Stimulus-correlated movements of the subject may lead to false results. The false information introduced by a movement by the subject is commonly referred to as a motion artifact. To accurately analyze the time series of images, motion artifacts must be removed by registering the series of images. Proper interframe registration of the time series of images to remove motion artifacts is particularly important in studies in that the subject's motion is an integral part of the experiment, such as experiments which require spoken responses or the performance of motor tasks. See, for example, J. V. Hajnal et al., Magn. Reson. Med., 31:283-291, 1994.
Techniques used to register image sets include the use of physical immobilization devices to maintain the subject in a known position and the placement of external fiduciary markers as landmarks for subsequent alignment. There are also numerous registration algorithms that construct "registered" images by repositioning each of the "misregistered" images by linear interpolation or Fourier regridding. See, for example, L. C. Maas et al., Magn. Reson. Med., 37:131-139, 1997, and A. Apicella et al., SPIE Medical Imaging III: Image Processing, 1092:252-263, 1989.
However, such registration algorithms can introduce additional artifacts into the image set that corrupt fMRI analysis.